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Full text of "Text Book Of Mechanical Engineering"

638

Reversing by Loose Eccentric.

(With lap and lead.} To assist the compression steam in
preventing a knock on the crank at the end of the stroke, it is
advisable that the valve be slightly open when the crank reaches
the dead-centre. This is called -lead, and is the amount of opening
of steam port at the commencement of the stroke. When a valve,
then, is provided both with lap and lead, the eccentric must lead
the crank by 90° + lap and lead, the lap only being apparent on the
valve, while both are apparent in eccentric position.*

Reversing" Gear.—Factory engines always rotate in one
direction, and thus only require a fixed eccentric. Again,
changing eccentric from H to j, Fig. 634, will change the direction
of motion, then shewn by the dotted arrow instead of by the full
arrow. Fig. 635 gives a means of moving the eccentric to the
opposite position, when the engine is at rest. Sheave B being
firmly bolted to a fixed plate A, can, on unloosing c, be slid from
h to j and rebolted, or, still further, can be made to take any
intermediate position between h and /, giving a variation of travel
with the same lap. Such decrease of travel means earlier cut-off,
as we shall see later.

Reversing by Loose Eccentric.—But it is not always
convenient to stop the engine for any considerable period, and
Fig. 636 shews one of many methods by which a single eccentric
may be quickly changed from one position to the other, c is the
crank, having a stop D fixed symmetrically, and A the eccentric
sheave, which, being loose; on the shaft, is provided with a balance
weight E to prevent spontaneous movement. At present the
sheave has its centre at j, and while the eccentric leads ttie crank,
the crank drives the eccentric ; so, although / causes the crank to
turn round left-handed, it is at the same time pushed before the
crank by the stop D. But the sheave may be swung round to
/ or h> when starting the engine, in a manner to be described
Lifting the gab F from the valve spindle pin. disconnects eccentric
from slide valve K, when the latter may be moved by the hand
lever H. On starting, then, the left hand lifts the handle GV while
H is moved by the right hand, and thus steam may be admitted
at will to either side of the piston, according to the direction in

* The student must carefully distinguish between the two applications of
the term 'lead,' which need not, however,